Candidates preparing to take CompTIA’s Network+ exam (N10-006) must have a strong understanding of the Domain Name Service (DNS) network service. This topic is covered as part of the Network+ exam’s “Network Architecture” knowledge domain. While many people are somewhat familiar with DNS from their use of the Internet, IT professionals need to expand this knowledge to include the different types of DNS records.

DNS is the network service used to assign fully qualified domain names (FQDNs) to IP addresses. DNS enables websites to have easy-to-remember names like examforce.com, rather than requiring visitors to type a unique IP address into a web browser to open a site. Computers can keep DNS records cached locally so that domain name – IP address matches can be performed quickly. If a query isn’t found in the cache, a request gets sent to an external DNS server for resolution.

This basic summary covers how DNS works, but doesn’t drill down into the different DNS records. Here are the main DNS records you need to know for the Network+ exam.

A (address) records

The most common DNS record is the A record, where A stands for address. An A record maps a host’s domain name to an IP address. For example, an A record for a server could be:

host.domain.com. A 192.168.0.1

CNAME (canonical name) records

A CNAME record is used to set up a DNS alias that points to a different domain which has an existing A record. Let’s say you want files.domain.com to be an alias that goes to host.domain.com, which has an A record pointing to the IP address 10.0.0.5. The CNAME record would be:

files.domain.com. CNAME host.domain.com.

MX (mail exchanger) records

MX records are used to indicate a mail exchange server responsible for accepting and routing messages for a domain. One important characteristic of MX records is that they must point to an A record, and not a CNAME alias.

You can identify multiple mail servers for a domain by using multiple MX records. You can even indicate a level of preference for each mail server by entering a priority number in each server’s MX record. In the following example for messages sent to username.domain.com, the mail servers will be prioritized from top to bottom:

MX 10 mail1.domain.com

MX 20 mail2.domain.com

MX 30 mail3.domain.com

PTR (pointer) records

A PTR record is the opposite of an A record. An A record maps a host’s domain name to an IP address; a PTR record switches this around. PTR records are primarily used to provide for reverse DNS lookups, which are commonly used by mail servers.

A reverse DNS lookup uses a special domain: in-addr.arpa. Here is an example of a PTR record:

192.168.0.1.in-addr.arpa PTR host.domain.com

AAAA (IPv6 address) records

DNS A records are used with IPv4 addresses; AAAA records perform the same function for IPv6 addresses. An AAAA record maps a host’s domain name to an IPv6 address, as follows:

host.domain.com. AAAA 2001:0db8:0000:0000:0000:ff00:0042:8329

There are several DNS record types we haven’t mentioned, but the ones covered in this article are the ones you will want to know for the Network+ exam.

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